ns a # sy) Sra SER asp ea See PE TNO og, nik, we 
EE eae 
364 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
in all but two of the known genera in both families of three equal plates. 
The orientation of these plates is uniform throughout both families, viz. : 
the interbasal sutures are directed toward the anal plate and the two 
antero-lateral radials. The anal plate in the Batocrinide is heptagonal, 
and is followed by three plates in the first 
ie Me interbrachial row, viz.: a second anal and two 
CL], interbrachials ; whereas in the Actinocrinids 
Ont % SJ HugB the anal plate is hexagonal, and is followed 
10 OLE (br. : 
An) ; () a no second anal. We regard the presence of 
SOS ® TOs a second anal within the first row of inter 
only by the two interbrachials, — there being 
VOS 4 (ib) Ay brachials as of considerable morphological im- 
ey DFS a US POs 
ee Se a portance. It is represented without exception 
i Gee from the Lower Silurian to the close of the 
1 ahh PO ae Devonian in all monocyclic Camerata in which 
fest cehe Sv eR me anal plates occur; in the Kinderhook, for 
plate. i = supplementary anal. idr = the first time, we find both forms together: 
Boca the Actinocrinoid with but two plates above 
the first anal, and the Batocrinoid with three. 
When there are three basals, the superior edge of the basal cup is nine- 
angled, six of the angles being salient, the others retreating, and the suture 
lines are directed to the middle of the two antero-lateral radials and the 
anal plate. By this arrangement two of the radials are heptagonal, and the 
three others hexagonal. In Compsocrinus with four basals, the posterior and 
anterior basals are pentangular, the two others quadrangular, the anterior 
radial hexangular, all others heptangular. 
The alternate bifurcation of the arms from the two main branches of 
the rays, which is characteristic of the Actinocrinide, forms another well. 
marked distinction between the two families, and is also very constant. 
Among the Batocrinide this mode of bifurcation occurs only in Genn«o- 
crinus from the Upper Devonian, but there it goes along with a second 
anal; and we must regard this genus as a transition form toward the 
Actinocrinide. The paired arm structure also, although not a constant 
character, occurs only among the Batocrinide; never among the Actino- 
crinide. Another significant fact, tending to confirm the separation of the 
two groups as distinct families, is that so far as known the Batocrinide 
disappear in Europe with the dawn of the Carboniferous — except perhaps 
